The present invention relates to a passivated optical device and a method of passivating optical device facets; more particularly, to providing a two-step in situ process including etching the facet surface and directing a passivating flux toward the facet to provide the necessary protection.
Semiconductor lasers that comprise a laser cavity defined by two parallel laser facets are well-known in the art. Among these are Fabry-Perot lasers and distributed feedback (DFB) lasers.
Despite much effort aimed at improving the reliability of lasers with facets, such lasers frequently are subject to early performance degradation and/or laser failure. This problem is particularly severe in high power lasers, e.g., in lasers that would otherwise be well-suited as pump lasers for optical fiber amplifiers in optical communication systems. The origin of this early performance degradation/failure can be attributed to a process involving a cascade of events and in particular carrier recombination in fault-containing zones of the facets, a reduction in the width of the forbidden band in these zones due to the heating caused by the carrier recombination, and, finally, increasing optical absorption of the radiation emitted by the laser due to the reduction in bandwidth. These phenomena contribute to accelerated heating of the facets of the laser and lead finally to irreversible destruction of the laser facets, often referred to as xe2x80x9ccatastrophic optical damagexe2x80x9d, or xe2x80x9cCODxe2x80x9d. Furthermore, the dielectric facet coating itself or the impurities in the coating can react with the semiconductor laser facets in the presence of light, heat and bias energies, resulting in facet degradation.
It is known that laser performance degradation can be reduced by provision of a contamination-free facet, followed by in-situ application of a passivation layer. U.S. Pat. Nos. 5,063,173 and 5,144,634 disclose that the passivation layer consists of Si, Ge or Sb, and that the passivation layer is deposited in situ onto a contamination-free laser facet. U.S. Pat. No. 5,171,717 discloses apparatus for cleaving semiconductor wafers in a vacuum system to minimize facet contamination. An alternative passivation technique, disclosed in U.S. Pat. No. 5,260,231, requires the inclusion of a sulfur-containing layer interposed between the facet surface and a passivation layer.
These and other prior art techniques of facet passivation are considered to put relatively severe requirements on the manufacturing process in terms of needing an ultra-high vacuum (UHV), high temperature, special fixturing, or any combination of the above. A need remains, therefore, for a passivation technique that is relatively simple and inexpensive, yet provides a laser facet of improved quality.
The need remaining in the prior art is addressed by the present invention, which relates to a passivated optical device structure and a method of passivating optical device facets using a two-step in situ process including etching the facet surface and directing a passivating flux toward the facet to provide the necessary protection.
In accordance with the present invention, the native oxide resident on the facet surface subsequent to cleaving is removed by using a non-damaging etch. The etch is accomplished in a minimal vacuum environment (e.g., 1E-6 torr), using a molecular gas such as, for example, XeF2. Once the etch is nearing its completion, a flux of passivating material (such as Si or ZnSe) is directed toward the etched facet surface. In accordance with the present invention, the etch processing is then slowly ramped down and the flux ramped up until the etch is completely turned off. The flux will result in the creation of a passivation film of a desired thickness (for example, approximately 200 xc3x85). While maintaining the device in the vacuum chamber, an in situ deposition of a protective covering film, such as any suitable oxide (e.g., SiO, SiO2, Al2O3, Ta2O3, or any combination thereof), is formed to completely cover the passivation layer.
In a preferred embodiment of the present invention, the covering film is formed using a benign process, such as thermal evaporation. Various other films besides oxides may be used as the outer covering layer, as long as the material provides the desired optical reflectivity requirements. The opposite laser facet may be similarly passivated by following the procedures of the present invention as outlined above.